# Engineers Corner > Chemical Engineering Design of vapor-liquid separator (knockout drum, flash drum)

## mbeychok

*In Metric units:*

A vapor-liquid separator drum is a vertical vessel into which a liquid and vapor mixture (or a flashing liquid) is fed and wherein the liquid is separated by gravity, falls to the bottom of the vessel, and is withdrawn.  The vapor travels upward at a design velocity which minimizes the entrainment of any liquid droplets in the vapor as it exits the top of the vessel. 

The size a vapor-liquid separator drum (or knock-out pot, or flash drum, or compressor suction drum) should be dictated by the anticipated flow rate of vapor and liquid from the drum. The following sizing methodology is based on the assumption that those flow rates are known.

Use a vertical pressure vessel with a length-to-diameter ratio of about 3 to 4, and size the vessel to provide about 5 minutes of liquid inventory between the normal liquid level and the bottom of the vessel (with the normal liquid level being at about the vessel's half-full level).

Calculate the vessel diameter by the Souders-Brown equation to determine the maximum allowable vapor velocity:

*V = (k) [ (dL - dV) / dV ]^0.5*

where:
V = maximum allowable vapor velocity, m/sec
dL = liquid density, kg/m3
dV = vapor density, kg/m3
k = 0.107 m/s (when the drum includes a de-entraining mesh pad)

Then A, the cross-sectional area of the drum, in m2 = (vapor flow rate, in m3/s) / (vapor velocity V, in m/s)

and D, the drum diameter, in m =  ( 4 A / 3.1416 )^ 0.5




> The GPSA Engineering Data Book recommends the following k values for vertical drums with horizontal mesh pads (at the denoted operating pressures):
> 
>     0 barg:  0.107 m/s
>     7 barg:  0.107 m/s
>   21 barg:  0.101 m/s
>   42 barg:  0.092 m/s
>   63 barg:  0.083 m/s
> 105 barg:  0.065 m/s
> 
> ...


The drum should have a vapor outlet at the top, liquid outlet at the bottom, and feed inlet at somewhat above the half-full level. At the vapor outlet, provide a de-entraining mesh pad within the drum such that the vapor must pass through that mesh before it can leave the drum. Depending upon how much liquid flow you expect, the liquid outlet line should probably have a level control valve.

As for the mechanical design of the drum (i.e., materials of construction, wall thickness, corrosion allowance, etc.), use the same methodology as for any pressure vessel.

Milt Beychok
(Visit me at *www.air-dispersion.com*)

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## sripri

Very detailed explanation on vapor-liquid separator . It surely would help all reading this discussion. Thanks mbyechok for posting the same.

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## mailtoamol1

i need to design the vertical vapor liquid seperator for the following compositionof CNG
 compositon is 
Carbon Dioxide: 0.038
Methane: 0.91
Ethane: 0.0452
propane: 0.0055
n-Butane: 0.0002
n-Hexane: .0011

Critical temperaur and pressure are 
critical temperature: 201.301 K
critical pressure: 4714.462 kPa
Molecular weight: 17.97 kg/kg-mole

the problem with me is how to find the liquid density for the operating condition

Please advice

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## mbeychok

mailtoamol1:

You really must learn to express your problems better!!  Are those compositions expressed as mol percent for the CNG in the vapor phase? If not, what are the units?

I assume that you know how to calculate the gas density using the ideal gas law corrected by the compressibilty factor ... or by using some more sophisticated equation of state.

I hope you understand the the liquid density in the Souders-Brown equation *is not the density of the CNG as a liquid*. It is the density of any liquid that might be present with the gas stream ... perhaps water or perhaps some heavier hydrocarbons. It may be a continuously present liquid or perhaps only an intermittent slug of liquid that may occur from time to time. You will have to work out that part for yourself.

What are your operating pressure and temperature and why did you present the critical pressure and temperature of the CNG?  Are trying to say that your operating conditions are close to the critical conditions of the gas? If so, then you may want to use a more sophisticated equation of state or other method to get the gas density  (rather than the ideal gas law and compressibility factor).

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## mailtoamol1

Thanks for reply
i am giving my problem in detail

i need to design the vertical vapor liquid seperator for the following composition of CNG
 in terms of mole fraction is

Carbon Dioxide: 0.038
Methane: 0.91
Ethane: 0.0452
propane: 0.0055
n-Butane: 0.0002
n-Hexane: .0011

operating temperature: 325.15 K
operating pressure: 25108.8kPa 

Critical temperaur and pressure are 

critical temperature: 201.301 K
critical pressure: 4714.462 kPa
Molecular weight: 17.97 kg/kg-mole

the problem with me is how to find the liquid density for the operating condition

Please advice

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## mbeychok

mailtoamol1:

What liquid are you talking about?  As I said before, the liquid in the Souders-Brown equation *is not the CNG* for your case. It is whatever liquid that you are trying to separate from the CNG, such as water or occasional slugs of higher molecular weight hydrocarbons.

Your operating conditions are obviously above the critical conditions ... so you will also have a problem of determining the gas density, as I also said before. 

Sorry, I can't help you any further. I doubt that the Souders-Brown equation even applies at conditions above the critical point.

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## dancho

Hi, I was wondering if anyone has experience with oils slugs in natural gas distribution piping. The line is too small to pig, so field workers have been blowing off gas through a small drum to purge the line, but this is potentially dangerous and messy. I have been researching into knock out drums and I want to size it down for portable use, possibly fitting it with a mist eliminator and a flame arrestor.  

Has anyone had the similar problem? or knows of a product/solution used in industry? Thanks!

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## amir_jkh2002

hi, i was wondering if anyone has experience about location of knock out drum. generally, it is located near the flare boom, but in our project flare is higher than wellhead facility, therefore, we have negative slop toward K.O. Drum. what should i do about it?

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## erranjana15

Hi,
i want to know that how we calculate the liquid density and gas density in the given composition of the feed?

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## nazir3003

Thanks a lot for the information to design a flash drum separator.
i want to design a gas-liquid separator. i followed your steps but how can i find the cost of such a separator. Please inform me about the costing.

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